Photographic process



United States Patent-O PHOTOGRAPHIC PROCESS Hendrik Jonker, Roelof Jan Hendrik Alink, and Tijs Willem van Rijssel, Eindhoven, Netherlands, assiguors to Hartford National Bank and Trust Company, Hart- 1 ford, Conn., as trustee No Drawing. Application July 10, 1951, Serial No. 236,068 r Claims priority, application Netherlands July 25, 1950 13 Claims. 01. 9s--ss This invention relates to methods in which a photosensitive layer is exposed so as to produce therein an image which is subsequently intensified to form a clear vertedby exposure into one or more other substancesthe light-reaction prod hereinafter referred to as ucts-which are capable of reducing silver compounds while splitting off silver metal. Consequently, the in vention does not relate to the use of silver halide as a photo-sensitive material. A silver image which is weak or even not perceptible by the eye is produced in the layer--hereinafter referred to as the silver germ image-which is subsequently intensified by physical development. The term physical development" within the scope of the invention is to be understood to mean the growth of a weak metallic image to form an image of sufficient photographic'density, the image silver produced by reduction at least in part not originating. from rare metal ions locally present in crystallised connection. I The term rare metal is to be understood here tomean a metal which is classified above the copper inthe electrolytic voltage series of metals. It thus includes inter alia silver, mercury, gold andqthe platinum metals; in most cases silver is preferred.

In the physical development of silver germ images obtained by exposure of layers containing halogen silver, it has been possible to produce a thousand to a ten thousandfold intensification of the image, resulting in a considerable increase in blackening. Since the number of silver germs produced per unit-surface of the layer -is approximately proportional to the quantity of light strik-- 2,738,272 Patented Mar. 13,

ment, the silver germ image usually has only a small capacity of being intensified, so that apart from an improvement in blackening and in colour, the amount of light required for producing the final contrast is decreased little or none by physical development. From the investigation leading to the invention and in which use is made of the experience gained with layers containing diazoand mercury compounds, it appears probable that the reason for the low intensifiability of the silver germ image must not be sought in a'small yield of the primary light reactionthis is in the said cases of the same order as the yield of the primary light reaction with the silver halides-but that this small intensifiability is attributable to the low degree of dispersion of the silver germ image to be developed.

It has .now been found that the extent of intensification of the silver germ image by physical development and hence its degree of dispersion may advantageously be influenced by certain steps to be mentioned hereinafter, in regard to the medium used during the formation of the silver'germ image. Furthermore, it has been found that the photo-sensitive substances of such layers which enter into consideration for carrying out the said steps may be found among those photo-sensitive substances in which the normal redox potential of thelight-reaction or decomposition product, measured at pH=l, has a value higher than '0.1 volt and that measured at pH=13 has a-value lower than +0.2 volt. A pH-value of 1 may, in view of the properties of the material of the layerand/or the binder, be considered as extreme permissible value, whilst on the other hand the pH must, as a rule, not be higher than 13 in view of the conversions of the material of the layer and/or binder under the action of strong alkali. The term redox potential of the light-reaction product' is to 'be understood here to mean the potential assumed by an indifierent electrode, for example a platinum electrode, in a solution of the light-reaction product and of the product obtained therefrom upon oxidation by silver ions, measured with respect to the normal hydrogen electrode. The normal redox potential is that which is obtained in a solution in which the molecular analytic concentrations of the two said products are equal to unity. In practice it sufiices that the said concentrations are equal. If measurements at pH==1 and pH=13 are not possible due to instability of the compounds concerned, the values of the normal redox potential will be determined by extrapolation. Furthermore, it has been found that for applying the steps according to the invention, to be mentioned hereinafter, to layers containing the photo-sensitive substances defined above, it is necessary that the difference AE, between the silver potential measured in the solution by which the layer is provided with silver compounds and the normal redox potential of the light-reaction product measured under the conditions prevailing in the solution by which the layer is provided with photo-sensitive material, should greater probability that more grains contain germsca pable of being developed and that each grain contains more germs capable of being developed, so that a contrast of greater blackening is produced upon physical development.

It has been suggested that silver germ images produced by reaction of a light-reaction product with silver compounds should likewise be intensified by physical development.

It has been found,'that, when using physical developbe 0.8 volt at the most, this difference being at least 0.2 volt at pH=13. If the layer is subjected to a further treatment before exposure, this must be taken into account for determining the normal redox potential.

The term silver potential is to be understood here to mean the potential assumed by a silver electrode in a solution containing silver ions, measured with respect to the normal hydrogen electrode.

In regard to the polarity of the silver potential and the normal redox potential, it is mentioned that these poentials are called positive if the electrodes concerned constitute the positive terminals of the ganvanic cells which are obtained if the electrodes are combined with i the normal hydrogen electrode. Reference is made to a decrease in potential if the electrodes have a decreasing positive or an increasing negative value.

For determining the potentials, use may be made of the methods known from electro-chemistry. Using the values of the normal silver potential (+0.8 volt) and the coefficients of activity known from literature, the silver potentials may in most cases be calculated with the use of the Nernst formula:

EAg=0.80-0.06 pAg' being the silver potential, and pAg the negative logarithm of the activity of the silver ions. If it is necessary to determine the silver potential at a pH value higher than 7, use may conveniently be made of the formula for pAg:

pAg=pH-6ll8 so that:

EAg=0.80-0.06 (pH6.l8)

The value of the normal redox potential is in .the majority of cases dependent upon acidity (pH), the infinence of the pH value becoming manifest in that the normal redox potential decreases upon increase of the pH, whereas the analytic concentrations of the light-reaction product and its oxidation product remain unchanged. The extent of the decrease in normal redox potential per pH-unit, as a rule, varies between volt and 0.09 volt and is frequently 0.06 volt. It may furthermore have difierent values in one and the same redox system in different pH-ranges. Furthermore, the value of the normal redox potential is dependent upon the presence of substances capable of binding themselves to different extents with the components of the redox system.

'Many methods of determining the normal redox potential are known. If the equipment is available, normal redox potentials may simply be determined with the use of a dripping mercury electrode and a cathode-ray oscillograph (J. Boeke and H. van Suchtelen, Philips Technical Review 4 (1939), page 231). The oscillograph provides a rapid indication of the bend of the current-voltage curve of the mercury electrode. According to C. H. Miiller in Physical Methods of Organic Chemistry, vol. II (New York 1946) page 1115, the potential corresponding to this bend is equal to the normal redox potential of the redox system under investiga- 'tion. The desired pH-value is adjusted with the use of e buffer mixture. For determining the redox potential, In certain cases use may advantageously be made of redox indicators.

For a number of light-reaction products the relationship between pH and normal redox potential is known, thus for example for hydroquinone and p-amino-phenol. In such cases it is particularly simple to determine the normal redox potential at a given pI-I-value.

On the ground of a large number of comparative model tests in which care was taken to ensure that the values of the silver potential and of the normal redox potential during the formation of the silver germ image could be measured in solution with a suitable degree of accuracy, it has been found that the degree of intensifi: cation of the silver germ image by physical development is determined by the difference AE between the said two potential values. More particularly it has been found that it is possible to increase the intensifiability of the silver germ image by increasing the difference AE, which is achieved by providing for the silver potential to be increased and/ or the normal redox potential of the lightreaction product to be decreased during the formation of the germ image. a

However, not any increase in AB leads to the desired result of an increase in dispersion of the silver germ image. Thus, it has been found that no effect or .at least no attractive effect upon the photographic sensitivity of 'the layer can be found if the value of AB in a given layer, due to the steps according to the invention, is not increased above 0.2 volt during the formation of the silver germ image, so that the amount of light required to produce the final contrast is not decreased or decreased only slightly. Consequently, in those cases in which AE cannot be increased above 0.2 volt by the steps according to the invention, these steps naturally will not produce any effect. Such will always be the case if AE at pH==l3 is less than 0.2 volt. Furthermore, it has been found that a further increase of AE above 0.8 volt no longer results in a greater intensifiability of the silver germ image even though a certain action upon the gradation still remains possible. Consequently, the steps according to the invention produce no improved result when applied to photo-sensitive layers which satisfy the condition that the normal redox potential of the light-reaction product measured at pH=1 has a value higher than 0.1 volt and measured at pH=l3, has a value lower than +0.2 volt, and in which AE is already 0.8 volt without taking these steps.

Measurements of potential in the layer. are unsuitable in practice. Although in a fairly large number of cases-- for this, reference is made to the examples following hereinafter-and certainly by measurements in solution it may be ascertained, though perhaps with some approximation, to What extent AE has been increased dur' ingthe formation of the silver germ image, difliculties still occur in certain cases, which difficulties may be connected, for example, with the. ignorance of the course of the diffusion processes during the use of the steps according to the invention. However, if the starting ma terial is a photo-sensitive layer, in which the normal redox potential of the lightreaction product, measured at pH=1, has a value higher than 0.l volt and measured at pH=13, has a value of +0.2 volt, and if care is taken to ensure that without the use of the steps according to the invention, the difference AE of the silver potential, measured in the solution by which the layer is provided with silver compound, is 0.8 volt at the most, this difference being at least 0.2 volt at pH=l3, then it is al ways possible, after exposure of this layer, to take steps whereby the difference AE during the formation of the silver germ image has a higher value than would be the case without these steps, such that under conditions of kind and duration of the physical development which otherwise are unvaried, the amount of light required to produce the final contrast is reduced at least 3 times.

As previously mentioned, the investigation carried out in establishing the invention, has revealed that upon continued increased of the difference AE above 0.2 volt the intcnsifiability of the silver germ image, and hence the photographic sensitivity, reaches an optimum value above which a further increase of AE remains without effect. Since for the intensifiability of a silver germ image its dispersion, that is to say, the number of germs into which a certain amount of germ silver is divided, plays the most important part, it may be concluded that the number of germs produced per unit-surface upon continued increase of AE also increases. However, a relationship which generally holds good between AE and the number of germs produced cannot be given. This relationship is a particular one for any particular system.

' This seems to indicate that, in each individual case, dif- 'ferent complicating and competitive factors play a part, which'may consist in a greater or smaller growth of the germs first produced or a greater or smaller coagulation of these germs. However this may be, the gain in sensitivity and the sensitivity-limiting value which is obtained or is obtainable by an increase in AE may vary fairly considerably in each individual case. Nevertheless, fur ther investigation has revealed that an increase of AE above 0.2 volt always has an important effect, even if the reactions take place in a layer and not in solution, the optimum effect, as a rule, occurring at the maximum AE obtainable, while furthermore it has been found that for photo-sensitivesystems for which so great anincrease of AE is obtainable, a further increase of AE, when having attained the value of 0.8 volt, does not result in further improvement of the intensifiability of the silver germ image. of AB to values within the range described above results,

- at least in part, in an increased number of germs and phase by aiming at a highquantum yield,'or inthe phase of the image formation byincreasing the intensification, 3

to intervene in a third phase, an intermediate phase, that of the formation or the introduction of germs. The

These experimental facts show that an increase 7 moments of intervention may be different, since, as will Y ditions for the formation of germs after exposure. However, as an alternative, the silver compound may be introduced subsequently to exposure and the advantageous medium be realised before, after or at the same time as the exposure. r

A-method of reducing the normal redox potential of the light-reaction product consists in that before or during the formation of the silver germ image substances are introduced into the layer which bind themselves with the oxidation product of the light-reaction product obtained by the conversion between the light-reaction product and the silver ions, to a greater extent than with the light-reaction product itself. The means usedin this 'method naturally varywith the photo-sensitive substance used. If this-is, for example, a ferric salt, then pyrophosphates, fluorides, citrates and oxalates, for example, have the effect of reducing the redox potential of the lightreaction-product and are thus fundamentally serviceable. If the light-reaction product is, for example, hydroquinone, which is the case if. p-diazophenolisfusedas the photo-sensitive substance, the addition of sulphite to a line vapour or with arliquid, the pH-value of-which is I higher than that of the layer and' more particularly with a liquid the pH-value of which is at least 7.. Use is preferably made of bufier mixtures to enhance the reproducibility of the method. f.

If the silver germ image is produced by conversionof light-reaction products with the use of silver ions, which silver germ image is intensifiable insu fiiciently, the said technique provides a method'of improving the possibility of intensifying the silver germ image. Accordmg to th s method, the layer is treated in such manner that during the formation of the silver germ image the silver poten-' tial in the layer is increased--'which at a pH-value smaller than about 7 make take place by increasin'gthe silver-ion concentration in the layer-and/or the redox potential in'the layer is decreased, which may takev place either by increasing the pH in the layer or by binding the oxidation product of the light-decomposition product.

According to the rules mentioned 'hereinbefore, a larg number of, substances are fundamentally serviceable for the method heredescribed, an intensifiable germ image being provided in a layer. The use ofgthe'invention' requires that the photo-sensitive substance is adapted to be converted by radiation into a light-reactionproduct capable of reducing silver ions to form silver metah The use of the methodaccording to thei invention en be explained more fully hereinafter, the light reaction may be coupled to take place in the presence of the silver compound from which the latent image is produced by cellulose,-a pH-value up to 11 is permissible.

ables-such high intensifications of the .silver germ :image to be obtained that the required amount of light is up to 1000 times smaller than in the methods hitherto known.

However, in many cases difdculties arise in the use of the steps according to the invention and this chiefly in those cases in which, due to the steps taken, silver germs or silver compounds spontaneously capable of providing germs, and insoluble silver compounds are depositedat unwanted places. It is evident that such deposits are also intensified in the physical development, thus completely neutralising the contrast in an enveloping fog which frequently may have, a blackening even higher than 2.

. According to the invention, it has furthermore been i found that this fogging may he avoided in that the development of the silver germ image already produced is preceded by treatment of the layer with a liquid capable of dissolving silver metal and/or silver compounds. For this purpose a treatment with dilute oxidising acid, such .as nitric acid, or a solution containing ferricyanide and thiosulph'ate is particularly suitable: These liquids must react for a short time only and the concentrations of the dissolved substances'must not be unduly high, since otherwise .the silver germ-image is also dissolved. Considering the increase in sensitivity obtained after physical development, the silver germ image is attacked to a much smaller extent than the latent fog. It is a surprisingfact that under certain conditions it is not necessary to dissolve silver metal during treatment but that. it suffices to use a solvent for silver compounds by which silver metal is substantially not attacked. The use ofbaths dissolving silver metal will preferably be restricted to those cases in which such is unavoidable. These cases occur if thepH-value of the medium during the formation of the silver germ image has been" higher, at least temporarily than 11. i V

vIf, however, the pH-value duringthe formation of germs is not chosen unduly high, it suflices to utilise a solvent for silver compounds. Such is generally the case ifthe pH- value remains below 9,for example if paper serves as a support for the photo-sensitive material. This value is influenced by the kind of the constitutive material of the layer so that in certain cases, for example in regenerated The use of the method here described affords the advantage-that r substantially no silver germs are dissolved during the subsequent treatment, so that very little only of the gain in sensitivity is lost. For such a treatmentuse may be made, for example, of soluble salts of thiosulphuric acid, thiocyanic acid or sulphurous acid, preferably alkaline or ammonium salts, or ammonia and the like.

The concentrations of the baths mustnot be unduly low since otherwise the desired effect is not obtained. However, the concentrations must neither be unduly high and the reaction period of the baths must not be undulylong in order to restrict the attack of the silver germ image to a negligible minimum. As an example of a treatment which is frequentlysuitable we may mention here impregnation ofthelayer in a 5% sodium-thiosulphate solution inwater for 3 minutes. Otherwise it is advisable in each individual case to determine the suitable composition of the values by a single test.

According to the invention, it has been found thattrea ment of the layer containing the silver germ image, with liquid-s dissolving silver compounds usually leads to satisfactory suppression of fogging.

It is known that metallic images may be produced in a layer by distributing therein a photo-sensitive material the light-reaction product of which has a normal redox potential such that AE has reached the value of 0.8 volt already in the neutral medium utilised, so that an increase of AE produces little effect only or no effect at all. Thus, I it is known to bring photo-sensitive derivations of anthraquinone such as anthraquinone 2-/' -'disulphori-acid sodium,

together with gelatine, on paper, to expose the photo sensitive layer thus obtained behind a photographic nega- 7 tive and todevelop the resultant weak image by immersion for a short period in a 1% solution of silver nitrate in Water and subsequent treatment for a short time with a developer, containing l-methyl-amino 4-hydroxybenzene and acidified by acetic acid. Furthermore, it is known to manufacture metallic images in a layer containing methylene-blue by exposure of this layer, followed by impregnation in a 1% solution of silver nitrate in water and development. In this and similar layers containing highly reducing substances after exposure, the above-mentioned steps for increasing AE produce no effects since even without these steps AE attains or exceeds the value of 0.8 volt above which the dispersion is not further increased. However, in this and in similar cases it is nevertheless possible to make successful use of the technical rule here given by utilising a layer which is acid during exposure, since in many cases acid photo-sensitive layers will be preferred. Reasons for this are, for example, the higher stability of many photo-sensitive compounds in acid medium, the higher stability of acid layers containing silver compounds the course of the primary light-reaction whichin certain cases is more rapid in acid medium, and the'higher stability of greatly reducing light-reaction products in acid surroundingswith respect to atmospheric oxygen. in the layer are such that AE acquires a value suitable for satisfactory or even maximum intensification of the silver image-hence by impregnating the layer in a solution having a pH-value such that AE comes within the range between ().2 and 0.8 volta layer is obtained which is more serviceable than would be possible without this step, since it has been found that in such layers the phenomenon of so-called regression occurs to a smaller extent than if use is made of, for example, neutral layers. The term regression is to be understood to mean that the silver germ image is produced to an extent which is smaller as the time between exposure and formation of the silver germ image is longer. The consequences of this regression, viz smaller reproducibility of the final contrasts and smaller intensifiability of the silver germ image, may be counteracted by keeping the layer till the formation of the silver germ image at a low pH-value by giving the layer strong acidity, for example, providing the photosensitive substance. Since it is precisely due to the presence of this acid that the normal redox potential of the light-reaction product is comparatively high, the lightreaction product: obtained has a smaller tendency of being oxidised.

However, during the experiments carried out in establishing the invention, it has been found that after the use of lower pH-vajlues in the layer, for example, between 5 and 7, in certain cases fogging also occurs and this is presumably likewise due to insoluble silver compounds such as silver oxalate and silver diazotate, which fog may be avoided in the manner above described.

For use of the invention a lyophile layer may be utilised consisting, for example, of regenerated cellulose, superficially saponified cellulose ester or paper. Such a layer is sensitised, for example by impregnation in an aqueous solution containing the photo-sensitive material and, if desired, silver compounds and or substances the object of which is to adjust the pH in the layer to a given value. As an alternative, it is possible for a binder such as gelatine, together with the above-mentioned solution, to be dried on a support.

The layer thus obtained, which has been exposed and contains an amount of silver compound sufiicient for the formation of the silver germ image, is treated with a liquid having a pH-value higher than the initial pH-value of the layer, or the layer is treated with an alkaline vapour. If, as may be examined beforehand, fogging during development does not occur, the layer subsequent to the adjustment of the pH-value may be treated with a developer. If fogging does occur then development is preceded by a treatment with a fog-eliminating liquid as mentioned be- By providing that, after exposure, the conditions amber-a fore. As previously stated, fogging always occurs if the pH-value during one of the treatments is higher than about 7. At lower pH-va'lues fogging occurs only when using particular photo-sensitive compounds such as diazonium compounds and methylene-blue.

For ease of treatment, a layer which has been exposed and contains an amount of silver compounds sufiicient for the formation of the silver germ image may be treated with an alkaline liquid which also dissolves silver compounds from the layer in order to avoid fogging. A solution of thiosnlphate, the pH of which is adjusted to a value between 8 and 12 by the addition of buffering substances and/or alkalis is particularly suitable for this purpose.

it is also very simple to treat an exposed layer containing 'silver compounds with a solution of ammonia which can provide the desired alkalinity of the layer and also dissolve silver compounds.

The silver compounds required for the formation of the silver germ image may frequently be provided in the layer prior to exposure. This is effected in the simplest way by dissolving this compound in the liquid by which the layer is sensitised. In this case also use may be made of the above-mentioned simplification by combination of alkaline treatment and dissolution of silver compounds.

If the silver compound is introduced into the layer only after exposure, for example since otherwise in certain cases the photo-sensitive layer becomes less stable or less photosensitive, the treatments may be simplified by treating the layer, subsequent to exposure, with a solution of silver compounds which also raises the 'pH-value of the layer. By this method, which is preferably utilised, it is ensured that a sufficient resolving power is obtained even with soluble light-reaction products.

Physical development, such as is used according to the invention, consists in reaction of a solution containing a rare-metal compound and a reducing agent, upon the silver germ image. The rare metal compound commonly used is 'a silver compound, usually a silver nitrate, so that the final contrasts have a grey to grey-black colour. However, it is also possible successfully to make use of developers containing a compound of other rare metals such, for example, as mercury or platinum. The reducing agent being used is, for example, hydroquinone. Furthermore, in order to enhance stability or to regulate the rate of development, such developers usually contain further substances such as alkalis, acids and salts. A suitable developer contains, for example, in addition to a reducing agent and a silver salt, alkaline hydroxide and alkaline sulphite. However, in most cases use is made of develop? ers containing a reducing agent. silver nitrate and an acid. A developer containing citric acid is particularly suitable, the contrast then becoming of a neutral grey colour a short time after the beginning of the development so that contrasts of satisfactory colour may be obtained with the use of these developers, which contrasts have a lower gradation than those obtainable With the use of other developers. As a matter of fact, here and also in other physical developers the gamma increases as the time of development is longer.

In the establishment of the invention it has furthermore been found that control of the gradation of the final image within very wide limits is possible by suitable choice of the silver-ion concentration and of the pH-value during the formation of the germ. As a rule, the gradation obtained is higher as the silver concentration of the baths is increased. However, the pH-value also appears to act upon the gradation during the formation of the silver germ image. As shown in the examples following hereinafter, it is thus possible to obtain metallic images the gradation of which varies within wide limits, for example, between the values 0.5 and 10. A large number of photo-sensitive substances may serve within the scope of the invention. Suitable as such are,.for example, diazonium compounds carrying, in addition to the diazonium group, one or more 'substituents; preferably aminoor hydroxy-groups at the orthoorpara-place which are substituted or not.

The use of the photo-sensitive layer according to the invention, which layer contains a photo-sensitive compound having a light-reaction product of low solubility or incapable of diffusion, leads to the additional effect of an increased resolving power even if the layer is not provided with a silver compound until after exposure. This may be achieved by means known per se such, for example, as by the introduction into the photosensitive compound of an aliphatic chain comprising at least 8 carbon atoms.

Other serviceable photo-sensitive substances are inorganic photo-sensitiveions bound, if desired, in a complex manner and changing, due to exposure, to ions containing a large number of electrons and the light-reaction product of which satisfies the condition that its normal redoxpotential measured at pH=l, has a value higher than 0.1 volt and that measured at pH=13 has a value lower than +0.2 volt. Suitable examples of such substances are the aforementioned ferriand uranyl compounds.

Furthermore, photo-sensitive organic substances reduced by exposure and the light reaction product of which also satisfies the aforementioned condition are likewise suitable. Such photo-sensitive organic materials, are, for example, benzoquinone and its derivatives which, upon irradiation, change to multivalent' phenols. Other suitable photo-sensitive organic substances which have not been used hitherto in photography are colouring matter such as indophenols, indamines and indanilines, which are converted by exposure into their leuko compounds. Light-reaction products reducing more strongly are obtained, for example, by the exposure of colouring matter belonging to the azine-, oxazine-, thiazineand indigo classes. As mentioned hereinbefore, when using such photo-sensitive substances it is desirable that the exposed layer should be maintained acid as long as possible in order to counteract regression and enhance primary light-reaction. In order to increase as much as possible the intensifiability of the silver germ image, an increase of the pH-value in the layer subsequent to exposure is required according to the invention. This colouring matter is of interest for colour pho tography owing to its sharply-defined photo-sensitivity in the visible region of the spectrum. (FriedmamAmerican Photography 43, 253 (1949).)

An important use of the method is that in which a rare metalirnage and in situ a colour contrast is obtained with the use of colouring physical developers.

7 The following examples are given to illustrate the invention.

The colouring physical development can always be used.

Example I superficially saponified cellulose acetate foil is sensitised by impregnation in a solution containing 0.21 g./mol. of ferri-nitrate, 0.23 g./mol. of silver nitrate and 0.15 g./mol. of citric acid per litre water, and subsequently dried.

After exposure under a sensitometer with the use of a mercury-vapour lamp, the foil is treated for 0.5 minute in a solution containing 0.5 g./mol. of sodium lactate per litre water (ph=5).

Due to the treatment with the last-mentioned solution, AB is increased from 0.13 volt to 0.44 volt. The following may serve to explain this. Without the use of the said step AE=0.75-0.62 volt=0.13 volt. The value 0.75 volt is that of the silver potential of the sensitising solution measured in a solution of 0.23 g./mol. of silver nitrate and 0.15 g./mol. of citric acid per litre water; this'value is not varied due to the step concerned. The value 0.62 volt is that of the normal redox potential measured in a solution containing ferri- 'nitrate developer.

initrate and ferr'o-nitrate in' a common concentration of 0.21 g./mol. in a litre of 0.15 'g./mol. of citric acid solution in water. The use of the step according to the invention, hence by the treatment with a solution containing 0.5 .g./mol. of sodium lactate per litre water for 0.5 minute, has the effect of decreasing the normal redox potential from 0.62 to 0.31 volt. The latter value has been found by measurement on a solution containing ferrinitrate and ferro-nitrate in a common concentration of 0.10 g./mol. in a litre of 0.5 g./mol. of sodium lactate solution in water.

After washing, physical development takes place for 5 minutes in a 1% metol-3.5% citric acid-0.4% silver The metol is a sulphate salt of p methyl-aminophenal. A copy of the sensitometer of deep-black colour is obtained, whereas without physical development a very vague, yellow copy only is obtained by much longer exposure. The gain in sensitivity as compared with the case in which the lactate trea ment is omitted is at least a factor 30.

If, instead of utilising the above-mentioned foil, use

is made of paper provided with a gelatine layer, a cor-- responding result is obtained. I

Example II A film of superficially saponified cellulose acetate is impregnated in a solution containing 0.01% of methyleneblue and 0.1 n sulphuric acid and subsequently dried. The blue-green film which absorbs red light but which freely transmits blue and green radiation is exposed with the use of an incandescent lamp, treated with a solution of 0.5 g./mol. of sodium lactate .(pH=7.5) for 10 secouds, with a solution containing 2.5% of silver nitrate and- 0.5 11 sodium lactate (pH=7.5) for 30 seconds, washed in distilled water for 3 minutes and developed in a physical developer.containing.1% of metol, 3.5% of citric acidand 0.4% of silver nitrate for 4 minutes. The AE-value is increased to about 0.72 volt by the treatment with the bath of silver nitrate and lactate. If the steps accord ing to the invention are omitted, only slight blackenings are obtained by an exposure which is twenty times greater. However, according to the invention, a satisfactory black copyof good positive gradation is obtained. The light-reaction product of methylene-blue has a comparatively lowsolublity, ,so that a suflicient resolving power is obtainable despite the fact that the layer does'not cont'ain'any silver compound during ex posure. i

As an alternative, the photo-sensitive colouring matter used may be other thiazine colouring matter, such as Lauths violet, or azine colouring matter, such as Janus green, or oxazine colouring matter such as brilliant cresyllue.

In this'example and in all other examples use may alternatively be made of colouring physical development, for example in blue-green colour with the use of a developer containing 0.08% of dimethylamino-4-phenylaminenie trate, 5% of tartaric acid and. 0.4% of silver nitrate, to-

which 2 c'cs.of a solution of 10% of naphthol-l in ethanol is added per 25 ccs. The silver image may be removed with the use of a liquid containing potassium ferricyanide and thiosulphate.

Example III nitrate in ccs. of water (pH-16.5; AE thus varies from-.1 Subsequently, in

about 0.05 volt to about 0.33 volt). order to'prevent fogging, a treatment takes place for 3 minutes with a solution of 5% by weight of potassium rhodanide, followed, after washing in distilled water for 1 1 Kminutes, by physical development in a solution of .2 g. of Metol, 0.7 g. of citric acid and 0.4 g. of silver nitrate in. 100 ccs. of water. A black copy of the sensitometer isv obtained. The gamma value of the blackening curve of the copyis 2.7, the maximum blackening being greater than 3. If development takes place for 4 minutes instead of 8. minutes, the gamma value is equal to unity. With respect to the method according to German patentspecification 601,676, the sensitivity is two hundred times higher. If; physical development only takes place, only a very vague, stainy copy is obtained by a ten times longer exposure.

Example IV A film consisting of superficially saponified cellulose acetate is sensitised by impregnation for 2 minutes in a solution containing 2.67 gms. of p-d'iethylaminobenzenediazoniumboronfiuoride and 0.85 gm. of silver nitrate in 100 ccs. of water, and subsequently dried in air. The value AB is in this case about 0.1. The dry film is exposed behind a sensitometer with the use of a mercury-vapour lamp. The film is now treated with a 10% solution of sodium acetate (pl-l: 8.5) for l minuteso that the value of AB is increased to about 0.4 volt-and subsequently with a 10% solution of sodium thiosulphate in water for 3 minutes. After washing in water, physical development takes place for 8 minutes with a solution containing 0.2% of Metol, 0.7% of citric acid and 4% of silver nitrate, followedby washing and drying. A beautiful black copy of the sensitorneter is then obtained, the blackening curve of which has a gamma value of 1.3, the highest blackening being about 3.

If a copy of the sensitometer is made with the same diazonium compound by the method according to German patent specification 601,676, the gain in sensitivity obtained by. the use. of the invention is found to be approximately a factor 1000. By proceeding according to the known method, a brown copy having a gamma value of 0.4V and a highest blackening of 0.5 is obtained.

, if the sensitising liquid contains. 8.5 gms. instead of 0,85 of silver nitrate and if treatment. takes place, after exposure, with a 5% solution of sodium lactate (pII=7.5) and then with a 5% solution of potassium rhodanide, and if development takes place in the same manner as before, beautiful black copies are again obtained. The gamma value is 2.3 after development for 4 minutes and. 8 after development. for-8 minutes. The highest blackening in the. copy is then greater than 3. As compared with the method according to German patent specification 601,676, the sensitivity is 500 times higher. With, respect to. the case in which physical development only takes place after exposure, a gain in sensitivity of a factor 50 to 100 is obtained. In the latter case fogging frequently occurs in the physical development.

To illustrate the above, a table is added whichmay give an idea of the variation in the optimum pH of alkaline baths applied to the aforementioned foils, different concentrations of silver ions .being added, to the sensitising liquid. However, the figures vary by the action of other factors such as the reduction power of the light-reaction product and, for example, the acid concentration in the layer prior to treatment.

p m PH Silver-ion concentration in sensitising liquid:

0.01 n abt. 0.05 n abt. 9 0.1.n abt; 8 0.5 n abt. 7

Example V A film consisting of superficially saponified cellulose acetate is impregnated for 2' minutes ina solution containing 1.66 gms. of p-aminobenzenediazonitunboroniiuoridcv and 1.5. gms. of silver nitratein. 10.0 ccs.. of'water. The value of AEis in this case about 0.1. volt.

The dried film is exposed behind a sensitometer withthe use of a mercury-vapour lamp, then treated with a 1% soda solution (pH=l0.5) for 1 minute-so that AB is increased to about 0.43-subsequently in a 5% solution of potassium rhodanide for 3 minutes and, after washing, physically developed for 3.5 minutes in a developer containing 1% of Metol, 3.5% of citric acid and 0.4% of silver nitrate. After washing and drying, a brown-black copy of the sensitometer is obtained, the gamma-value of which is 1.8 and the highest blackeningof which is found to be 1.6. The gain in sensitivity with respect to the method of German patent specification 601,676. is. in this case a factor 100. The gain in sensitivity with respect to the case in which physical development only takes place, is a factor 20.

Example VI superficially saponified cellulose acetate is impregnated in a solution of 4 gms. of the chlorinebenzene sulphon-acid salt of p-diazophenol and 4 gms. of silver nitrate in ccs. of water. The value of AB is in this case about 0.16 volt. After exposure as in the preceding examples, a treatment for 1 minute takes place with a 10% soda solution (pH=10.8; AB is thus increased to 0.42 volt) and for 5 minutes in a 10% solution of sodium thiosulphate. After Washing for 7 minutes and development for 4 minutes in the physical developer of Example I, the gamma value is 0.5 and the highest blackening is l. The colour is brown-black, the gain in sensitivity with respect to the method of German patent specification 601,676 being a factor 150. The gain in sensitivity with respect to the method in which physical development only takes place, is a factor 10.

Example VII A film of superficially saponified cellulose acetate is impregnated in the solution containing 2.29 gms. of hydroxy-l, diazoniurn-Z, methyl-6,. benzenesulphonic acid- 4, together with 3.33 gms. of silver nitrate and 2 gms. of citric acid per 100 ccs. of water. After exposure, the film is treated with the following solutions: 10%. soda solution (pH: 10.6) for 1.5 minutes, 10% solution of potassium rhodanide for 4 minutes, physical development of Example I for 5 minutes. The result, is a black sensitometer copy having a gamma-value of 1.7 and at highest blackening of 2. The sensitivity is about 200 times that obtained by the method according to German patent specification 601,676 and also about 200 times that, which is obtained if the treatment with the two firstrmentioned solutions is omitted.

If the sensitising solution, instead of containing the above-mentioned diazonium compound, contains 2.67 gms. of diazoniurn-1, naphthol-Z, sulphonic acid-4, then after a similar treatment the gamma-value is 2.5 and the highest blackening is more than 3, the gain'in, sensitivity being of the same order of magnitude.

Example VIII Non-glued paper is impregnated in a solution of +2.5 gms. of amino-1 dimethoxy-2.5 benzene diazonium-boronfluoride-4 in 100 ccs. of acetane. Then again impregnation takes place in a solution of 1.7 gms. of silver nitrate and 2 gms. of citric acid in 100 ccs. of Water. The value of AB is in this case about 0.34. After drying and exposure, treatment takes place successively with a solution of sodium lactate (pH=7.5, so that AB is increased to about 0.66) for 1 minute and with a 5% solution of potassium rhodanide for 3 minutes, followedby development for 3 minutes in the physical developer of Example I. The sensitivity is in this case 50 times-as high as that obtained by the method according to German patent specification 640,233. The gamma-value is 1.3.

For superficially saponified cellulose acetate, isensitised in asstrong a solution of the diazonium compound in methanol and subsequently. impregnated in a solutionof 3.grns. of silver nitrate and 2 gms. of citric acidin 100 cos;v ofwater, to which a similar treatmentiisappliedias 13 to the paper, a sensitivity gain of 200 withrespect to the method according to German patent specification 640,233 hasbeenfound. The gamma value is 2.4 and the highest blackening is greater than 3. development was 5 minutes.

Example IX from about 0.04 to about 0.39 volt) and washed. This is followed by a-treatment with a solution of 0.37 g. of potassium ferricyanide and 1.25 gms. of sodium thiosulphatein100 ccs. of water. After renewed washing development takes place in a solutionof metol (0.5%), tartaric acid (1%) and silver nitrate (0.5%) for 12 minutes. The copy obtained is grey-black with a violet colourpthe gamma-value is 2.5 and the greatest blackening is more than 3. The sensitivity is about 70times as high as.that obtained by the method according to German patent specification 601,676.

vAnother piece is sensitised with a similar solution which, however, contains citric acid instead of tartaric aCldu ThIS is exposed for 5 seconds behind a photographicnegative at a distance of 25 ccs. from a 125 watt mercury vapour lamp. After washing in a 1% soda solution, thecopy is exposed to the action of 0.2 n nitric acid for -15 seconds and, after washing, developed in the developer of Example I for 6 minutes.

Thepositive copy obtained is of a beautiful blackness, of satisfactory sharpness and beautiful gradation. In this case anexposure sufiices which is 100 times-smaller than if the'same m'aterial is treated in accordance with German patent specification 601,676, or if physical development only takes place. In the latter case images of a high gamma value are obtained.

Example X Superficially saponified cellulose acetate issensitised with a. solution of 2.67 gms. of p-diethylaminobenzenediazoniumboronfluoride and 3.4 gms. of silver nitrate in 100Jccs. of water and dried. Subsequently, 4 pieces are exposed and each treated in one of four alkaline solutions of 15% sodium thiosulphate, obtained by adding 10% of sodium acetate, 3% of borax, 10% of soda and 1.7% of piperidine respectively. After washing, all four strips are physically developed for 8 minutes in a developer containing 0.2% of metol, 0.7% of citric acid and 0.4% of silver nitrate. The pH value of the alkaline thiosulphate solutions is 859.1, 10., and 12 respectively. The gammavalues of the copies are about 2, and the highest black- Example XI The film of the preceding examples is impregnated in a solution containing only.2.67% of p-diethylaminobenzene-diazoniumboronfluoride and subsequently exposed. After exposure, the film is treated with a solution containing. 5% of sodium lactate and 5% silver nitrate (pH=6.5) for 1 minute, subsequently with a 5% solution :of potassium rhodanide for 3 minutes and then physically developed in the developer of Example III for 1.0 minutes. A clear black image is produced having a gamma-value of about 0.5 and a maximum blackening which is slightly higher than 1. If use is made of a 15 times greater exposure and if the physical development is precededby treatment with a solution of silver nitrate in water exhibiting the same pAg as the above-mentioned solution of silver nitrate and sodium lactate, an image is obtained which is barely perceptible only.

The duration of the physical .114 I Example XII The sensitised film of Example IX is introduced, after exposure, into ccs. of 10% soda solution. After 1 minute, a further 20 ccs. of 12 n ammoniasolution is added to the bath, the film being left in the mixture for a further 3 minutes. Subsequently, slight washing takes place, followed by physical development in the developer of Example I. A beautiful black image of excellent gradation is obtained. Au unsufiiciently covered image of brown colour obtained according to German patent specification 601,676 requires about a 200-fold amount of light. If physical development only takes-place, an image is ob-' tained which is black and well-covered but hard. In this case, however, a 100-fold amountof light is certainly required.

Proceeding in a different manner, the film, after the soda treatment, is introduced into 35 ccs. of a 2% solu tion of ammonium rhodanide in water. After being left in this bath for 4 minutes, 5 ccs. of each of the following three solutions are added whilst stirring the liquid:

(a) 10% of ammonium rhodanide and 1% of silver nitrate in water;

(b) 2% of ammonium rhodanide, 2% of metol and 0.5% of sodium sulfite in water;

(c) 2% of ammoniurnrhodanide, 5% of soda and 3.8% of potassium hydroxide in water.'

After being left in the physical developer thus produced for 20 minutes, an image of violet-grey colour is obtained. i Y

Example XIII I A foil of superficially saponified cellulose acetate is sensitised by impregnation for 2 minutes in 0.1 n nitric acid in which 0.75% of benzoquinone and 0.5% of silver nitrateare dissolved, and subsequently dried.-

Exposure behind a sensitometer with the. use of a mercury vapour lamp is followed by treatment with 0.5 11 solution of ammonia in water for 2 minutes. After washing, development takes place for 2 minutes in a developer consisting of 1% of metol, 3.5% of citric acid and 0.4% of silver nitrate in water. A satisfactory, neutral-grey copy of weak gradation is obtained. The gain in sensitivity with respect to the cases in which the ammonia treatment and the physical development respectively are omitted, is certainly a factor 25 to 30. The value AB is here decreased by the treatment with ammonia from 0.05 to about 0.04 volt.

If a foil of superficially saponified cellulose acetate is impregnated in a saturated solution of toluylene-blue in distilled water,'in which furthermore 1.5% of silver nitrate is dissolved, and if after exposurebehind a negative with the use of a mercury vapour lamp the foil is treated in the manner as indicated above for the benzoquinone foil, physical development for 2.5 minutes yields a satisfactory fogless copy of neutral-grey to black colour and satisfactory positive gradation. Without physical development, and also if physical development only takes place, much longer times of exposure provide only vague images.

In the latter case, exposure may alternatively take place by X-rays, behind a template. Other indamines, indophenols, indanilines, for example, thynol-indophenol and phenol-blue may be used as a photo-sensitive system in a similar manner.

Example XIV Strips of the exposed film of Example I are treated for 0.5 minute in one of the following solutions:

(a) 7.5% of potassium sodium tartrate and 3% of borax in water (pH=9.3);

(b) 5% of sodium pyrophosphate in water (pH: 10.1

(c) 10% of soda in water (pH==10.8).'

Subsequently, the strips are treated with a 0.3 n solution of ammonia in water forZl minute. After washing,

development takes place in the'developer of Example 1' for 5 minutes. Satisfactory images are obtained, the gain in sensitivity with respect to the cases in which the steps for increasing AE during the formation of germ are omitted, certaiulybeing a factor 100. AB is increased by the. treatment with: one of the. solutions a, b and c from about 0.13 to about. 0.73 volt.

Other strips are washed for l to 2 minutes in the following solutions:

(d) 0.3 n ammonia in water (pH=11.4);

(e) 10% of soda and of sodium thiosulphate in water (pH=.10.8).

After washing and physical development as indicated above, satisfactory results arev likewise obtained, although the gain in sensitivity is now only a factor 25 to 60.

Example XV A film of superficially saponified cellulose acetate is sensitised by impregnation in a solution containing 0.25 g./m ol. of uranylnitrate, 0.23 g./rnol. of silver nitrate and 0.08 g./mol. of citric acid per litre water, and subsequently dried.

After exposure under a. sensitometer with the use of a mercury vapour lamp, a strip is treated in the solution (b) of Example XIV for 0.5 minute, subsequently bathed in a solution of sodium thiosulphate in water for 3 minutes and after washing for 7 minutes, physically developed in the developer of Example I for 5 minutes. Another strip is treated in the solution (c) of Example XIV for 3 minutes. After washing, this strip is also developed in the same developer for 5 minutes. Satisfactory, black copies of the sensitometer are obtained. The gain in sensitivity with respect to the method in which physical development only is used is a factor 100 and 25 respectively.

What we claim is:

l. A method of producing photographic contrasts comprising the steps, selectively exposing a layer having a pH less than about 7.0 and containing a light sensitive compound selected from the group consisting of diazonium, azine, oxazine, thiazine and indigo compounds, indophenols, indamines, and indanilines to decompose said compound at selected portions of the layer, said layer after exposure having a AB, in the presence of a water-soluble silver salt, of less than 0.8volt, treating the exposed layer with an alkaline medium and moisture in the presence of said water-soluble silver salt to form a latent silver contrast, the alkalinity of the alkaline medium being sufiicient to increase AB to a value greater than at least 0.2 volt, and physically developing the latent contrast by then applying to said layer a developingsolution containing a salt of a metal higher than copper in the electromotive series and reducible to metal in solution and a watensoluble reducing agent.

2. A method of producing photographic contrasts comprising the steps, selectively exposing a layer having a pH less than about 7 .0 and containing a' light sensitive oxazine compound to decompose said compound at selected portions of the layer, said layer after exposure having 21 AB, in the presence'of a water-soluble silversalt, of less than 0.8 volt, treating the exposed layer with an alkaline medium and moisture in the presence of said water-soluble silver salt to'form a latentsilver contrast, the-alkalinity of the alkaline medium being sutficicnt to increase AE to a value greater than'at' least 0.2 volt, and physically developing the latent contrast by'then applying to said layer a developing solution containing a salt of a metal higher than copper in the electromotive series and reducible to metal in solution andawater-solublc reducing agent.

3. A method of producing photographic. contrasts comprising the steps, selectively exposing slayer having a pH less than about-10am containinga light sensitive diazoni m mpo nd tel-d composesaid compoun t lected portions of thelayer, said layerafter exposure hav? ing a AB, in thexpresence ofa water-soluble silver salt, of=lessthan 0.8 volt, treating-theexposed layer with an alkaline medium and moisturein the presence of said water-soluble silver salt to form a latent silver contrast, the alkalinity of the alkaline medium being sufiicient to increase AE to a value greater than at least 0.2 volt, and physically developing the latent contrast by then applying to said layer a developing solution containing a salt of a metal higher than copper in the electromotive series and reducible to metal in solution and a water-soluble reducing agent.

4. A method of producing photographic contrasts comprising the steps, selectively exposing a layer having a pH less than about 7.0 and containing a light sensitive thiazine compound to decompose said compound at selected portions of the layer, said layer after exposure having a AB, in the presence of a water-soluble silver salt, of less than 0.8 volt, treating the exposed layer withan alkaline medium and moisture in the presence of said water-soluble silver salt to form a latent silver contrast, the alkalinity of the alkaline medium being sufficient to increase AE to a vaue greater than at least 0.2 volt, and physically developing the latent contrast by then applying to said layer a developing solution containing a salt of a metal higher than copper in the electromotive series and reducible to metal in solution and a water-soluble-reducing agent.

5. A method of producing photographic contrasts comprising the steps, selectively exposing a layer having a pH less than about 7.0 and containing a light sensitive indamine to decompose said compound at selected portions of the layer, said layer after exposure having a AB, in the presence of a water-soluble silver salt, of less than 0.8 volt, treating the exposed layer with an alkaline medium and moisture in the presence of said water-soluble silver salt to form a latent silver contrast, the alkalinity of the alkaline medium being sufficient to increase AB to a value greater than at least 0.2 volt, and physically developing the latent contrast by then applying to said layer a developing solution containing a salt of a metal higher than copper in said electromotive series and reducible to metal in solution and a water-soluble reducing agent.

6. A method of producing photographic contrasts comprising the steps, selectively exposing a layer having a pH less than about 7.0 and containing a light sensitive azine compound to decompose said compound at selected portions of the layer, said layer after exposure having a N5, in the presence of a water-soluble silver salt, of less than 0.8 volt, treating the exposed layer with an alkaline medium and moisture in the presence of said water-soluv ble silver salt to form a latent silver contrast, the alkalinity of the alkaline medium being sufficient to in: crease AE to a. value greater than at least 0.2 volt, and physically developing the latent contrast by then applying to said layer a developing solution containing a salt of a metal higher than copper in the electromotive series and reducible to metal in solution and a water-soluble 'reducing agent.

7. The method of claim 1 in which the silver salt is added to the light sensitive layer before exposure of the layer.

8. The method of claim 1 in which the reducible salt in the developing solution is a silver salt.

9. The method of claim 1 in which any silver or insoluble silver compounds formed in unexposed areasare dissolved before subjecting the system to physical developing.

10. A method of producing photographic contrasts comprising the steps, selectively exposing a layer having a pH less than about 7.0 and containing a light sensitive compound selected from the group consisting of diazonium, azine, oxazine, thiazine and indigo compounds, indophenols, indamines, andindanilines, to decompose said compound at selected portions of the layer, said layer after exposure having a AB, in the presence of a watersoluble silver salt, of less than 0.8 volt, treating theex posed layer with an aqueous solution of an alkali metal thiosulfate in the presence of said water-soluble silver salt to form a latent silver contrast, the alkalinity of the thiosulfate solution being sufficient to increase AB to a value greater than at least 0.2 volt, and physically developing the latent contrast by then applying to said layer a developing solution containing a salt of a metal higher than copper in the electromotive series and reducible to metal in solution and a water-soluble reducing agent.

11. The method of claim 10 in which the solution of the thiosulphate is adjusted to a pH value of about 8 to 12.

12. The method of claim 1 in which the alkaline medium is ammonia.

13. The method of claim 1 in which theralkaline medium is an alkaline thioeyanate.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF PRODUCING PHOTOGRAPHIC CONTRASTS COMPRISING THE STEPS, SELECTIVELY EXPOSING A LAYER HAVING A PH LESS THAN ABOUT 7.0 AND CONTAINING A LIGHT SENSITIVE COMPOUND SELECTED FROM THE GROUP CONSISTING OF DIAZONIUM, AZINE, OXANINE, THIAZINE AND INDIGO COMPOUNDS, INDOPHENOLS, IMDAMINES, AND INDANILINES TO DECOMPOSE SAID COMPOUND AT SELECTED PORTIONS OF THE LAYER, SAID LAYER AFTER EXPOSURE HAVING A $E, IN THE PRESENCE OF A WATER-SOLUBLE SILVER SALT, OF LESS THAN 0.8 VOLT, TREATING THE EXPOSED LAYER WITH AN ALKALINE MEDIUM AND MOISTURE IN THE PRESENCE OF SAID WATER-SOLUBLE SILVER SALT TO FORM A LATENT SILVER CONTRAST, THE ALKALINITY OF THE ALKALINE MEDIUM BEING SUFFICIENT TO INCREASE $E TO A VALUE GREATER THAN AT LEAST 0.2 VOLT, AND PHYSICALLY DEVELOPING THE LATENT CONTRAST BY THEN APPLYING TO SAID LAYER A DEVELOPING SOLUTION CONTAINING A SALT OF A METAL HIGHER THAN COPPER IN THE ELECTROMOTIVE SERIES AND REDUCIBLE TO METAL IN SOLUTION AND A WATER-SOLUBLE REDUCING AGENT. 